1. Field of the Invention
The present invention relates to a polymer electrolyte fuel cell (PEFC) for a vehicle, a household stationary power supply and a mobile device. Furthermore, the present invention relates to a membrane electrode assembly (MEA) used in such a PEFC.
2. Description of the Related Art
Fuel cells are power generation systems which produce electric power along with heat. A fuel gas including hydrogen and an oxidant gas including oxygen reacts together on electrodes containing a catalyst so that the reverse reaction of water electrolysis takes place in a fuel cell. Fuel cells are attracting attention as a clean energy source of the future since they have advantages such as a small impact on the environment and a low level of noise production relative to conventional power generation systems. Fuel cells are divided into several types according to the employed ion conductor. A fuel cell which uses an ion-conductive polymer membrane is called a polymer electrolyte fuel cell (PEFC).
Among various fuel cells, a PEFC, which can be used at around room temperature, is considered as a promising fuel cell for use in a vehicle and a household stationary power supply etc. and is being developed widely in recent years. A joint unit which has a pair of electrode layers on both sides of a polymer electrolyte and which is called a membrane electrode assembly (MEA) is arranged between a pair of separators, on each of which a gas flow path for either supplying a fuel gas including hydrogen to one of the electrodes or supplying an oxidant gas including oxygen to the other electrode is formed, in the PEFC. The electrode for supplying a fuel gas is called a fuel electrode, whereas the electrode for supplying an oxidant gas is called an air electrode. Each of the electrodes includes an electrode catalyst layer, which has stacked polymer electrolytes with carbon particles on which a catalyst such as a noble metal of platinum group is loaded, and a gas diffusion layer which has gas permeability and electron conductivity.    <Patent document 1> JP-A-2004-158387    <Patent document 2> JP-A-2008-041406
In a PEFC, a fuel gas including hydrogen is supplied to the anode electrode and an oxidant gas including oxygen is supplied to the cathode so that an electromotive force is generated. The following reactions occur on the anode electrode catalyst layer and the cathode electrode catalyst layer.On the anode: H2→2H++2e−On the cathode: (1/2)O2+2H++2e−→+H2OAt this time, the protons generated on the anode are transferred to the cathode via the polymer electrolyte membrane while the electrons move via the external circuitry.
Water is produced in the cathode catalyst layer. This reaction occurs since catalysts are at a three-phase interface. The three-phase interface is an interface in which three phases, namely, pores through which materials are transferred (a path for gases), the polymer electrolyte membrane which conducts ions (a path for protons), and the catalyst loaded particles which conduct electrons (a path for electrons) are involved. There is a problem in a fuel cell that the power generation performance decreases due to a flooding phenomenon, which occurs in such a way that the water produced by the reaction fills in the three-phase interface and disturbs the power generation reaction. The influence of the flooding phenomenon is particularly serious in the case where the fuel cell operates at a high current density and thus a large amount of water is produced.
In addition, there is another problem concerning the protons generated in the anode electrode catalyst layer. Since the protons must be ultimately transferred to the cathode via the polymer electrolyte membrane from the anode catalyst layer, resistance by the anode catalyst layer to the proton transfer causes a problem of a decrease in the battery performance.